Thermal timing apparatus



Oct. 22, 1963 k R. L. BAKER EI'AL THERMAL TIMING APPARATUS Filed June24. 1960 2 Q 6 Ne. w .1 u. m 0

TEMPERATURE FIG.4

INVENTORS ROBERT L. BAKER By ROBERT E. PROUTY WOW AGENT United StatesPatent 3,103,166 THERMAL TIMING APPARATUS Robert L. Baker and Robert E.Prouty, Logansport, Ind, assignors to Essex Wire Corporation, acorporation of Michigan Filed June 24, 1960, Ser- No. 38,623 1 i 4Claims; (Cl. 200-122) This invention relates to thermal timin apparatusand more particularly to thermal time delay switches of the type whichemploy bimetallic elements responsive to an electric heater toactuateswitch contacts.

In devices of this type, the heater is energized from a voltage sourcewhich may have substantial voltage fluctuations. Unless provisionis madeto compensate for the variation in heating rate resulting from. suchvoltage fluctuation, the operating time of the thermal time delay switchis not precise enough for many applications. It is, therefore, one ofthe objects of the present invention to provide thermal timing apparatusof the electrically heated bimetal type that is not appreciably affectedby variations in voltage of the heater.

Another object of this invention is to provide an improved thermaltiming apparatus of the foregoing character which is of simpleconstruction and few parts and which may be easily and inexpensivelymanufactured.

The thermal timing apparatus constructed in accordance with thisinvention comprises generally a first bimetal member having asubstantially uniform deflectiontemperature curve and a second bimetalmember having a deflection-temperature curve which reverses at apredetermined temperature. An electric heating element is provided forheating both bimetal members 'to cause op eration of switch contacts ina predetermined time interval by the diiferential action of the bimetalmembers. The bimetal members are so disposed in relation to each otherthat when initially heated they both act in a manner tending to operatethe switch contacts. However, when the second bimetal member is heatedto the temperature at which its deflection reverses, it will act tooppose operation of the switch contacts. By constructing the apparatusto operate with energizing voltages for the heating element which causethe second member to be heated to a temperature exceeding that at whichthe second member reverses its deflection, the effectivecontactoperating action of both members are proportional to theenergizing voltage but in opposition to each other. Thus thedifferential action of the bimetal members is made substantiallyindependent of voltage variations to provide a substantially constanttime delay.

Other objects, advantages and features of the invention will be apparentfrom the following description and the accompanying drawing in which:

FIG. 1 is a plan view of a thermal time delay switch embodying thepresent invention;

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a sectional view of a modified embodiment of the switch ofFIGS. 1 and 2; and

FIG. 4 is a graphic illustration of the deflection-temperaturecharacteristics of the bimetals used in the switches of FIGS. 1, 2 and3.

The thermal time delay switch shown in FIGS. 1 and 2 of the drawing hasa stack which includes an insulator 10, a bimetal strip 11, an insulator12, a bimetal strip 13, an insulator 14, a metal terminal 15, aninsulator 16, a metal terminal 17 and an insulator 18. This stack ismounted in the open end of an enclosure consisting of a metalchannelshaped case 19 with an end wall 26 and an insulatingiplate 21supported upon the stack and the edge of end wall 2%). Four ears 22formed integrally with case 19, are bent over plate 21 to clamp theswitch parts in assembled relation. The case and stack parts may haveinterfitting apertures and projections to insure their permanentalignment.

A first electrical contact 23 is attached to strip 11 near the free endthereof and a second electrical contact 24' is attached to strip 13 nearthe free end thereof. A heater coil 25 of insulated wire is wrappedaround strip 13 in close heat transfer relation thereto. Strip 11 facesthe heater coil 25 and also responds to heat emitted therefrom. Theheater coil 25 is electrically connected to terminals 15 and 17 forenergization from a suitable power source.

The disposition and specific material of the bimetal strips 11 and 13are important features of the present invention. Thedeflection-temperature characteristics of the bimetal strips are shownin FIG. 4 where 11a is the deflection curve of strip 11 and 13a is thedeflection curve of strip 13. It will be noted that the material ofstrip 11 deflects at a substantially constant rate throughout thetemperature range in which it operates. Many of the commonly usedbimetal materials such as those composed of superposed layers ofironnickel alloys of different compositions have a substantially uniformdeflection curve within a certain temperature range. of strip 13 has avariable deflection curve and first deflects in one direction until acertain temperature is reached but at higher temperatures reverses itsdirection of deflection. Such materials are also well known and for aspecific example reference is made to U.S. Pat..

an initial increase in temperature from normal ambient temperatures, thestrips will deflect toward each other..

The heater coil 25 is suitably designed to heat strip 13 to at least thetemperature at which its deflection reverses when a specified minimumoperating voltage is applied to coil 25. The dimensions of the strips 11and 13 are preferably chosen so that contacts 23 and 24 will engageafter heater coil 25 has been energized for a desired time period at theminimum operating voltage.

It will be apparent that if a voltage higher than the minimum operatingvoltage is applied to heater coil 25, the increased heat delivered bycoil 25 will cause bimetal strip 11 to deflect toward bimetal strip 13at a higher rate. Strip 13 is also heated faster when the voltage ishigher and will more quickly reach the temperature at which itsdeflection reverses. Before contacts 23 and 24 can engage, bimetal strip13 will reverse its direction of deflection and deflect in the samedirection that strip 11 is deflecting. This increases the distance whichstrip 11 must deflect before contact 23 engages contact 24 and thuscompensates for the increased deflection rate of bimetal strip 11. Sincethe increased rate of deflection of strip 11 and the increased distancestrip 11 has to deflect to cause engagement of contacts 23 and 24 areboth proportional to the increase in heater coil voltage, the timerequired for contacts 23 and 24 to engage can be made to besubstantially independent of voltage variations over a wide range. Thusthe thermal time delay switch according to the present invention isuseful to control a variety of equipment such as air conditioningequipment, which may operate under varying voltage conditions.

In FIG. 3 there is shown a normally closed contact switch which isgenerally similar to that illustrated in FIGS. 1 and 2 and correspondingparts are designated by like reference numerals. In the embodiment ofFIG. 3, however, the bimetal strips 11 and 13 are supported with theirsides reversed from that previously described, so that their directionsof deflection with temperature change are reversed. The bimetal strips11 and 13 are stressed toward each other and it will be apparent thatthe effect The material of temperature upon strips 11 and 13 will besimilar to that previously described except that the temperature changeinitially changes the pressure exerted upon contacts 23 and 24 by thestrips 11 and 13 instead of causing their movement. When the strips havebeen heated sufficiently to overcome the initial biasing forces, theywill act to separate contacts 23 and 24.

7 While the invention has been illustrated and described in itspreferred embodiments and has included certain details, it should beunderstood that the invention is not to be limited to the precisedetails herein illustrated and described since the same may be carriedout in other ways falling within the scope of the invention as claimed.

What is claimed is:

' 1. A voltage-compensated thermal time delay switch comprising incombination: a first bimetal member having a substantially uniformdeflection-temperature curve; a second bimetal member having adeflection-temperatur e curve that reverses at a predeterminedtemperature; electric heating means mounted in heat transfer relationwith said members to cause said members to move upon being heated;switch contacts mechanically connected with said members and operatedthereby in a given predetermined time interval in response toenergization of said heating means; said first and second members beingdisposed to deflect in opposite directions with an increase intemperature of said members when the temperature of said second memberis below said predetermined value; said first and second members beingdisposed to deflect in the same direction with an increase intemperature when the temperature of said second member is above saidpredetermined value; said heating means being eflective to heat saidsecond member to a temperature above said predetermined value in saidgiven time interval when a voltage is applied to said heating meanshaving a magnitude greater than a first selected value; said first andsecond members being effective to operate said contacts only when saidsecond member is heated to atemperature exceeding said predeterminedvalue; and said members being responsive to variations in the heatingrate of said heating. means to maintain substantially constant the timeinterval in which said operation of said switch contacts occurs during'variations in the magnitude of the voltage applied to said heating meansbetween said first selected value and a second selected value greaterthan said first selected value.

2. A thermal time delay switch as specified in claim 1 wherein saidbimetal members comprise two strips arranged so as to extend generallyin the same direction in spaced relation to each other, said firstmember being supported at one end in fixed relation to the adjacent endof said second member, said switch. contacts comprising first and secondelectrical contacts mounted respectively on said first and secondmembers and arranged tomove into and out of contact with each other inresponse to relative movement between said members, and said electricalheating means comprising an electrical heating element associated withsaid second member.

3. A thermal time delay switch as specified in claim 2 wherein saidcontacts are normally separated, and said bimetal members being sodisposed in relation to each other that when heated by said heatingmeans they deflect toward each other until the temperature of saidsecond member reaches said predetermined temperature.

4. A thermal time delay switch as specified in claim 2 wherein saidbimetal members are stressed to normally urge said contacts inengagement with each other, and said bimetal members being so disposedin relation to each other that when heated by said heating means theyboth act to decrease the pressure exerted on said contacts by saidmembers until the temperature of said second member reaches saidpredetermined temperature.

References Cited in the file of this patent

1. A VOLTAGE-COMPENSATED THERMAL TIME DELAY SWITCH COMPRISING INCOMBINATION: A FIRST BIMETAL MEMBER HAVING A SUBSTANTIALLY UNIFORMDEFLECTION-TEMPERATURE CURVE; A SECOND BIMETAL MEMBER HAVING ADEFLECTION-TEMPERATURE CURVE THAT REVERSES AT A PREDETERMINEDTEMPERATURE; ELECTRIC HEATING MEANS MOUNTED IN HEAT TRANSFER RELATIONWITH SAID MEMBERS TO CAUSE SAID MEMBERS TO MOVE UPON BEING HEATED;SWITCH CONTACTS MECHANICALLY CONNECTED WITH SAID MEMBERS AND OPERATEDTHEREBY IN A GIVEN PREDETERMINED TIME INTERVAL IN RESPONSE TOENERGIZATION OF SAID HEATING MEANS; SAID FIRST AND SECOND MEMBERS BEINGDISPOSED TO DEFLECT IN OPPOSITE DIRECTIONS WITH AN INCREASE INTEMPERATURE OF SAID MEMBERS WHEN THE TEMPERATURE OF SAID SECOND MEMBERIS BELOW SAID PREDETERMINED VALUE; SAID FIRST AND SECOND MEMBERS BEINGDISPOSED TO DEFLECT IN THE SAME DIRECTION WITH AN INCREASE INTEMPERATURE WHEN THE TEMPERATURE OF SAID SECOND MEMBER IS ABOVE SAIDPREDETERMINED VALUE; SAID HEATING MEANS BEING EFFECTIVE TO HEAT SAIDSECOND MEMBER TO A TEMPERATURE ABOVE SAID PREDETERMINED VALUE IN SAIDGIVEN TIME INTERVAL WHEN A VOLTAGE IS APPLIED TO SAID HEATING MEANSHAVING MAGNITUDE GREATER THAN A FIRST SELECTED VALUE; SAID FIRST ANDSECOND MEMBERS BEING EFFECTIVE TO OPERATE SAID CONTACTS ONLY WHEN SAIDSECOND MEMBER IS HEATED TO A TEMPERRATURE EXCEEDING SAID PREDETERMINEDVALUE; AND SAID MEMBERS BEING RESPONSIVE TO VARIATIONS IN THE HEATINGRATE OF SAID HEATING MEANS TO MAINTAIN SUBSTANTIALLY CONSTANT THE TIMEINTERVAL IN WHICH SAID OPERATION OF SAID SWITCH CONTACTS OCCURS DURINGVARIATIONS IN THE MAGNITUDE OF THE VOLTAGE APPLIED TO SAID HEATING MEANSBETWEEN SAID FIRST SELECTED VALUE AND A SECOND SELECTED VALUE GREATERTHAN SAID FIRST SELECTED VALUE.